A white polyester film is widely used as a base material of a printing record reception sheet for ink-jet printing, thermal printing, offset printing or the like. This white polyester film is generally produced by having a polyester contain inorganic fine particles or an incompatible resin.
In recent years, due to improvements in printing precision and the sharpness of a printed material, a white polyester film which appears more sophisticated has been demanded. To fulfill the demand, a white polyester film resulting from adding two or more types of inorganic fine particles to a polyester and a white polyester film resulting from adding a combination of inorganic fine particles and incompatible resin to a polyester are proposed in JP-A 4-153232 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A 6-322153. However, these techniques are also subjected to demand for further improvements in sharpness and fanciness. Particularly, in an internally illuminating billboard used for displaying the name of a store such as a convenience store or advertising merchandises, the number of fluorescent lamps as light sources is increased to increase the brightness of the surface of the billboard to 1,000 lux or higher, or colorful neon tubes are used to make a design drawn on the billboard more noticeable, so as to increase an advertising effect. The internally illuminating billboard uses a reflecting plate for increasing illuminance. Further, for liquid crystal displays that have generally employed a backlight system which sheds light from the back of the display in lighting the display surface, a sidelight system as described in JP-A 63-62104 has been increasingly widely used owing to an advantage that it can be thin and light the display surface uniformly. The sidelight system is a system which sheds light of cold cathode tubes or the like from the edges of an acrylic plate or the like having a certain thickness. Because of dot printing, illuminating light is dispersed uniformly, and a screen having uniform brightness is obtained. According to the system, the display can be made thinner than that adopting the backlight system because lights are placed not at the back but at the edges of the screen.
To prevent illuminating light from entering the back of the screen, a reflecting plate must be placed on the back of the screen. The reflecting plate must be thin and have high light reflectance. As a white polyester film for a liquid crystal display reflecting plate which matches this purpose, a white polyester film containing titanium oxide is known because it is easy to handle and inexpensive. However, as described in JP-B 8-16175 (the term “JP-B” used herein means “Examined Japanese Patent Publication”), the white polyester film having mere addition of an incompatible resin or titanium oxide has a limitation on an improvement in reflectance, and it cannot be said that the brightness of the screen is satisfactory.
An improvement in reflection efficiency can be expected when inorganic particles such as titanium oxide are added in high concentration. However, for example, when 50 wt % of the inorganic particles are added, ruptures often occur and film formation becomes very difficult or even impossible in some cases due to the very high particle concentration. Further, as a realistic measure for decreasing the frequency of occurrence of the ruptures when the particle concentration is very high, it is conceived to copolymerize a polyester resin. In this case, however, the thermal shrinkage becomes very high, and only a film showing extremely poor thermal dimensional stability can be obtained.